Electricity + Control July 2018

ENERGY MANAGEMENT + ENVIRONMENTAL ENGINEERING

Solutions for Cape Town water crisis

Hennie Pretorius, Endress+Hauser

South Africa, a water scarce country, should consider its water supply as its most valuable resource. All South Africans with access to piped water should always be responsible water users, and not only when a crisis looms, as is currently the case in the Western and Eastern Cape regions.

Take Note!

Cape Town has three main options to augment its water supply in times of drought: Large aquifers in the City 1

and the Cape Flats. The desalination of abundant sea water. Direct potable reuse.

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W e focus on Cape Town and the Western Cape in order to indicate what possible solutions there are for drought strick- en areas. The Cape Town region experiences a Mediterranean climate with warm dry summers and winter rainfall. It is dependent on water that comes from mostly six large dams of which Thee- waterskloof is by far the largest. Since 2013 the stored volume of water has slowly been decreas- ing but three consecutive years of extremely low rainfall has accelerated the crisis. The period 2015 – 2017 is regarded as the driest three-year period in more than 80 years, and 2017 was the region’s driest year since 1933. Modelling by consultants indicates that this is a ‘one in 400-year’ event. Cape Town has three main options to augment its water supply in times of drought, with the first being the large aquifers in the City and Cape Flats regions: The Cape Flats aquifer, theTable Mountain aquifer and the Atlantis aquifer. They can deliver, as per early estimates, 80, 40 and 30 Megalitres per day respectively. This water is, for the most part, treated in conventional water treatment plants. The second option is desalination of abundant sea water. This process uses membrane technolo- gy to remove the salt from the water and to deliver potable water. It is, however a costly method be- cause of the high energy demand. Desalination process overview • Sea water is drawn from the sea through pipe- lines and enters the plant through screens to filter out larger material. • Pre-treatment filters, which may include ultra- filtration, remove smaller particles. • The filtered sea water is pumped to the Re-

verse Osmosis building where it is pushed through RO membranes at pressures of more than 60 bar. • These membranes remove the salts and only the water molecules are let through. • The ultra-pure water is then demineralised and disinfected to comply with local drinking water standards. • This potable water is stored in tanks from where it is pumped into the water distribution network. • The salt concentrate, known as brine, is re- turned to the ocean. Source: Sydneydesal.com.au The third option is direct potable reuse. This is de- fined as the injection of recycled water into the potable water network once it has been through a traditional water treatment plant, or into the raw water supply before passing through the tradi- tional water treatment plant. Many people would like direct water reuse to be limited to agricultural or industrial use. Unfortunately, with the current and future water emergencies, using it as potable water would be essential. The purified municipal waste water is taken through various membrane filters and finally through RO membranes to get to the required quality. This is a viable solution, and unavoidable in future, for inland drought strick- en areas such as Gauteng. Beaufort West’s water supply during the current drought has relied on its direct potable reuse plant. The City of Cape Town will have to get the cor- rect blend of water supply mix that will be cost-ef- fective during times of plenty but can step up to

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Desalination technology is

widely used within the South African mining industry.

Electricity + Control

JULY 2018

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